FIG. 1 illustrates the transmission scheme used in Direct Sequence Spread Spectrum communications (DSSS). A stream of data may be represented as a series of symbols, with each symbol being characterized by a symbol interval Ts. For example FIG. 1(a) illustrates two, square wave symbols.
Each data signal in a DSSS communication scheme is associated with a spreading sequence denoted as s(t) and shown in FIG. 1(b). A spreading sequence, s(t), is a pseudo-random noise sequence, which is unique to each data signal d(t). A spreading sequence s(t) consists of a series of rectangular pulses (or chips) of duration Tc and which typically have magnitude +1 or −1, however, other magnitudes are also possible.
As is shown in FIG. 1c, a DSSS signal x(t) is the product of the data stream d(t) with the spreading sequence function s(t). This operation is a modulation in a classic sense, i.e. d(t) modulates s(t). From another point of view, the spreading sequence function is used for spreading the data sequence, which results in the spread spectrum signal x(t). The ratio of Ts/Tc is called a spreading factor, which is the number of chips in one symbol period.
In Direct Sequence Spread Spectrum Code Division Multiple Access Systems (DSSS-CDMA or DS-CDMA), each user's data, for example a digitized phone call, is spread over a fixed bandwidth made available by the wireless service provider. Multiple calls are superimposed on top of each other in the frequency domain with each call assigned a unique spreading sequence defined by its code. A CDMA data stream may then be despread by a receiver, such as a mobile handset or a base-station, by reference to the spreading code assigned to the data.
A DSSS signal broadcast from a transmitter (i.e. a base-station or a handset) may reach a receiver via different paths—referred to as multipaths—due to the refraction and reflection of the signal by objects along its path. Since each multipath signal travels along a different path, each signal arrives at the receiver at a different time creating what is called a temporally diverse signal. FIG. 2 shows a temporally diverse signal model for three, hypothetical multipaths 1. αi and τi refer to the complex magnitude and multipath delay, respectively, for the ith multipath ( also known as the ith tap).
If a DSSS wireless receiver also employs multiple antennas, and each antenna is separated sufficiently apart that the signals received by the two antennas are uncorrelated, the signal components may be characterized by a space variable (referred to as spatial diversity) as well as a time variable (referred to as temporal diversity). The signal components have spatial diversity because the antennas are separated-i.e. are located at different locations in space. Spatial diversity can also be created by using antennas with different polarizations. In multi-antenna DSSS receivers, these signal components must be combined. In the art, the combination of temporally diverse signal components is referred to as RAKE combining; the combination of spatially diverse components is referred to as antenna combining.
RAKE-combining
The structure of Direct Sequence Spread Spectrum (DSSS) communication allows a receiver to resolve multipaths and combine them. In this scheme, different multipaths are demodulated separately by so called RAKE fingers. Traditional RAKE combining is performed on a single antenna and only provides temporal diversity. Proakis, J. G., Digital Communications, McGraw-Hill, New York, 1995. This reference and all other references are hereby incorporated by reference. A typical rake finger consists of a number of correlators (despreaders) each operating on a different DSSS data signal. In some cases, as many as 4 correlators may be present each working on a different data-signal or time off-sets of the same data-signal. FIG. 3 depicts a simplified rake finger 3, comprising two correlators 5 operating on a DSSS signal comprising a pilot signal 7 and a data signal 9, each spread by its own PN sequence. The rake finger correlates the input signal with the appropriate PN sequence (this process is referred to as despreading) to generate the pilot and data symbols. The output of each rake finger is then multiplied by a combining coefficient and the resulting products are summed.
FIG. 4 shows a typical RAKE combiner and logic for combining L DSSS Pilot/Data channels 7, 9, each with an offset, or time delay of (n−nL) chips and each characterized by a spreading PN sequence. In a typical first step, the pilot and data signal channels 7, 9 are despread. In a typical second step, the L RAKE fingers 3 determine L RAKE combining coefficients, xL, 11, from the L DSSS pilot signals 7. In a typical third step, each of the L RAKE combining coefficients 11 are then used to weight 13 the corresponding data signal channels 9. In a typical fourth step, the RAKE combiner may then sum 15 the weighted data signals 9.
There are a number of current methods for calculating RAKE combining coefficients. The most common method for determining the combining coefficients used in conventional DSSS receivers is the maximum ratio combining method (MRC). Proakis, J. G., Digital Communications, McGraw-Hill, New York, 1995. In this method, the RAKE fingers are co-phased and scaled proportional to the signal-to-noise ratio, SNR, on each finger. In practice, the conjugate of the channel tap, also known as the multipath component estimate, is used as a measure of the SNR on each finger. An MRC combining coefficient may be expressed as:WMRCk=conj(αk)  Equation 1
Where WkMRC is the coefficient of the kth finger and αk is the kth multipath component (tap) of the wireless channel. The actual computation or estimation of the channel coefficients can be accomplished in a number of ways including the use of the pilot channel, or data directed approaches.
Another common method for determining the RAKE combining coefficients is the minimum mean-squares error (MMSE) method. In MMSE, the combining coefficients are optimized so as to minimize the mean of the square error. MMSE combining provides a theoretical maximum performance for a RAKE receiver. An optimized MMSE solution is shown in Equation 2. Alireza Tarighat and Babak Daneshrad, Performance Analysis of Different Algorithms for CDMA2000 Antenna Array System and a New Multi User Beamforming (MUB) Algorithm, Proceedings of WCNC'00, Chicago, September 2000; S. Haykin, Adaptive Filter Theory, Prentice Hall, 1996.WMMSE=ξRxx−1rxpRxx=E(XXt)  Equation 2rxp=E(Xp*)
WMMSE is an optimized RAKE coefficient vector. X is an L-element (L is the number of fingers) vector representing the output of the pilot correlators, shown in FIG. 4, and p* represents the complex conjugate of the known pilot symbols used as the desired signal at the receiver. R−1xx is the inverse of the autocorrelation matrix, Rxx, formed by taking the expectation value of the matrix XXt. Xt is the hermetian adjoint of X. rxp is the expectation value of the cross correlation of x with the complex conjugate of the expected pilot symbol p*. Although it is not practical to directly implement this algorithm, it does provide an upper bound on a RAKE receiver's performance for comparison purpose.
In practice, MMSE methods are rarely solved analytically, rather iterative approximations such as the method of Least Mean Squares (LMS) or the Recursive Least Squares (RLS) methods are used with LMS especially preferred because of its straightforward implementation. S. Haykin, Adaptive Filter Theory, Prentice Hall, 1996.
Antenna Combining
In the early days of wireless communications, antenna combining was traditionally associated with flat fading channels. A flat fading channel is one where the impulse response, h(t), of the channel is described by a single impulse with time varying amplitude and phase (hflat-fading(t)=α(t)δ(t−τ)ejθ(t)). Antenna combining can also be defined for frequency selective channels. Such channels are characterized by an impulse response, h(t), consisting of multiple impulses each with time varying amplitude and phase (hfreq-selective-fading(t)=Σαl(t)δ(t−τi)ejθi(t)).
A number of methods exist in the art for antenna combining, including switched selection combining, equal gain combining, maximum ratio combining (MRC), and minimum mean square error (MMSE) combining. A. F. Naguib and A. Paulraj, Recursive Adaptive Beamforming for Wireless CDMA, Proceedings of ICC95, Seattle, pp. 1515–19; Proakis, J. G., Digital Communications, McGraw-Hill, New York, 1995; Alireza Tarighat and Babak Daneshrad, Performance Analysis of different Algorithms for cdma2000 Antenna Array System and a New Multi User Beamforming (MUB) Algorithm, Proceedings of WCNC'00, September 2000.
When performing MRC antenna combination over a flat-fading channel, the MRC antenna combining coefficients may be expressed asWMRC=conj(h)  Equation 3
where WMRC represents the MRC antenna combining coefficient and h represents the channel tap vector. conj(h) represents the complex conjugate of h. The channel tap vector is a K element vector, where K is the number of antenna channels (one tap for the channel seen on each antenna). The elements of h are complex numbers and may be determined from each antenna channel using pilot aided, decision directed, or other channel estimation techniques that are well known in the art.
When performing MMSE antenna combining over flat fading channels, the MMSE antenna combining coefficients may be expressed as:WMMSE=ξRxx−1rxp  Equation 4
where WMMSE represents the optimized MMSE antenna combining coefficient, ξ is a scaling constant. R−1xx is the inverse of the autocorrelation matrix, Rxx. Rxx, is formed from the expectation value of the matrix X Xt, where X is a K element vector containing the output of the pilot correlators, Xt is the hermetian adjoint of X, and K is the number of antennas. rxp is the expectation value of the cross correlation of X with the complex conjugate of the expected pilot symbol p*. In addition, WMMSE may be determined based upon the common pilot channel using other methods well known in the art.
RAKE-combining and Antenna Combining
Both RAKE combining and antenna combining are necessary when using multiple antennas for communication over frequency selective channels. FIG. 5 illustrate the two, current, state-of-the-art methods for implementing RAKE combining and antenna combining in a receiver. B. H. Khalaj, A. Paulraj, T. Kailath, Spatio-Temporal channel estimation techniques for multiple access spread spectrum systems with antenna arrays, IEEE International Conference on Communications, Seattle, 1995; Joseph C. Liberti, and Theodore S. Rappaport, Smart Antennas for Wireless Communications, Prentice Hall, 1999. In FIG. 5a, the spatially diverse signal components associated with the different antennas are first combined via antenna combining and then the temporally diverse signal components are combined via RAKE combining. In FIG. 5b, the combination order is reversed with RAKE combining preceding antenna combining. Still, in either method, the combination of temporally diverse signal components is done in serial with the combination of spatially diverse signal components.
FIG. 6 shows a combined hardware/logic schematic for a typical current, state-of-the-art RAKE/Antenna combining method as applied to a dual antenna receiver and as typified in U.S. Pat. Nos. 5,809,020 and 5,812,542. In FIG. 6, each antenna 17, 19 receives 4 multipath signal components. In a first step, each multipath signal component is seaparated into pilot 7 and data signal 9 components by despreading the received signal by reference to the PN sequence. In a second step, the pilot components 7 from antenna 1 17 are extracted by RAKE fingers 1–4 21 and presented to the first coefficient calculation block 23. The first coefficient block 23 may calculate the first set of RAKE coefficients 24 using any of the known RAKE combining methods, such as the MMSE and MRC RAKE combining methods. The pilot components 9 from antenna 2 19 are separately extracted by RAKE fingers 5–8 25 and are presented to the second coefficient calculation block 26. The second coefficient calculation block 26 in turn produces a second set of RAKE coefficients 27. In a third step, the first set of RAKE coefficients 24 produced by the RAKE coefficient calculation block 23 of antenna 1 17 are used to scalar multiply 29 the data components 9 (i.e. w1d1+w2d2+w3d3+w4d4) from antenna 1 17. Similarly, the second set of RAKE coefficients 27 produced by the second RAKE coefficient block 26 of antenna 2 19 are used to scalar multiply 31 the data components 9 (i.e. w5d5+w6d6+w7d7+w8d8) from antenna 2 19. The result of summing the weighted, despread data components from each of the antennas, is two complex scalars. In a fourth step, the two complex scalars produced from step 3 are antenna combined 33 before being input to the slicer 34. Since the result of summing the RAKE weighted data components is a complex scalar, the antenna combining step may effectively treat each complex sum as a flat fading antenna channel. Step 4 may be implemented with any of the antenna combining methods detailed above.
FIG. 7 shows a combined hardware/logic schematic for a typical current, state-of-the-art RAKE/Antenna combining method as applied to a dual antenna receiver. In FIG. 7, each antenna 17, 19 receives 4 multipath signal components. In a first step, each multipath signal component is separated into pilot 7 and data signal 9 components in a despreading block 39, 41 by despreading the received signal with the appropriate PN sequence. In a second step, the pilot components 7 from antenna 1 17 are presented to a first antenna coefficient calculation block 43 and the pilot components 7 from antenna 2 19 are presented to the second antenna coefficient calculating block 45. Each antenna coefficient calculation block, 43, 45, may calculate a first and second set of antenna combining coefficients (a1, a2, a3, a4 and a5, a6, a7, a8) 44, 46, using any of the methods for antenna combining known in the art. In a third step, the first set of antenna combining coefficients 44 produced from the pilot components of antenna 1 17 are used to multiply 47 the pilot 7 and data components 9 from antenna 1 17. Similarly, the second set of antenna combining coefficients 46 produced from the pilot components of antenna 2 19 are used to multiply 47 the pilot 7 and data components 9 from antenna 2 19. In a fourth step, the pilot components 7 from each antenna are summed and the data components 9 from each antenna 17, 19 are summed 49. In a fifth step, the summed pilot components are used to produce a set of RAKE coefficients 52. In a sixth step, the RAKE coefficients 50 produced by the RAKE coefficient calculation block 50 from the two antennas 17, 19, are used to scalar multiply 51 the data components 9.
The methods according to the invention are based on the unexpected discovery that if an MMSE RAKE is performed on all the multipath components in a multi-antenna DSSS receiver, improved SNR performance may be obtained. Thus, in the preferred methods according to the invention, the RAKE combining steps and antenna combining steps illustrated in FIGS. 6 and 7 are performed in one step and in parallel.